In this type of applications, the objective is to produce steel parts that combine high mechanical strength, good corrosion resistance and good weldability. It must also be possible to fabricate these hot stamped parts using high productivity methods. These requirements apply in particular in the automobile industry, where the objective is to reduce significantly the weight of the vehicles. Anti-intrusion parts or parts that play a role in the safety of automotive vehicles such as bumpers, door reinforcements or center pillars, for example, require the qualities indicated above. These qualities can be obtained in particular thanks to steel parts, the microstructure of which is martensitic or bainitic-martensitic.
The fabrication of parts of this type is known in particular from publications FR2780984 and FR2807447, according to which a blank cut in a steel sheet for heat treatment, pre-coated with a metal or metallic alloy, is heated in a furnace and then hot stamped. The pre-coating can be aluminum or an aluminum alloy, zinc or zinc alloy. During the heating in the furnace, this pre-coating alloys with the steel substrate to form a compound that provides protection of the surface of the steel against decarburization and the formation of scalet. This compound is suitable for hot forming. Holding the piece in the tool after forming makes a rapid cooling possible, which leads to the achievement of hardened microstructures in the steel substrate, which are associated with good mechanical characteristics of strength and hardness. This method is known as “press hardening”.
In a method of this type, the blanks are generally heated in continuous furnaces, whereby the blanks are moved forward in these furnaces on rollers. This phase comprises a heating step followed by a temperature hold in the furnace, generally at around 900-950° C. The hold temperature and the hold time are a function of, among other things, the thickness of the blanks and the type of pre-coating on the blanks. For productivity reasons, it is desirable to employ methods that make it possible to shorten the step of heating in the furnace as much as possible. In this regard, publication EP2312005 discloses a method in which a coil of steel with an aluminum pre-coating is provided and is then annealed at 600-750° C. for a period of time ranging from 1 hour to 200 hours. A diffusion of the iron from the substrate to the pre-coating occurs, to the point where a pre-alloy product is obtained. After cutting, these pre-alloyed blanks can be heated more rapidly, in particular on account of the emissivity modification caused by the pre-alloying treatment. However, this method requires a prior, time-consuming annealing of the coil.
Document EP2463395 also proposes to accelerate the kinetics of the heating phase by locally reducing the reflectivity of a blank by means of various methods: prior application of black paint, modification of surface roughness by shot blasting, by rolling, laser or immersion etching in an acid solution. This document also describes examples in which pigments in the aqueous phase or polyester/melamine based black paint in a solvent phase have been deposited on galvanized pre-coatings. Considering the conventional mixing rate: 90-92% polyester and 8-10% melamine (C15H30N6O6) in the paint layer after drying and a maximum pigment concentration by volume of 30%, the nitrogen content in the paint layer used in these tests is on the order of 1.7 to 2.4% after drying. However, this method completely disregards certain essential problems linked to the later utilization of these parts. After hot stamping, the parts must be suitable for painting by cataphoresis, must be weldable and corrosion resistant. However, as will be shown below, the application of a conventional black paint that is resistant to high temperatures before hot stamping does not make it possible to obtain these properties.
Attempts have therefore been made to identify a method that makes it possible to simultaneously increase the productivity of the hot stamping process and to control the process so that the hot stamped and hardened parts thus obtained are compatible with conventional industrial production conditions, i.e. they do not require, for example, a modification of the existing settings of resistance spot welding machines for the assembly of these parts. This method must also be compatible with the fabrication of welded blanks pre-coated with an aluminum coating that requires prior ablation of a portion of the pre-coating on the periphery of the blank, as described in document EP2007545.
Attempts have also been made to identify a method that is relatively insensitive to certain potential variations of the fabrication conditions. In particular, attempts have been made to devise a method, the results of which are relatively insensitive to the preparation conditions of the pre-coated blank.
Moreover, a method is sought that will make it possible to obtain excellent resistance to delayed cracking. It is known that press hardening makes it possible to obtain parts with extremely high mechanical strength, the microstructure of which can be sensitive to cracking due to the presence of hydrogen in the presence of stresses, for example those resulting from the cutting of the parts. Attempts have therefore been made to define a method that does not present any increased risk of delayed cracking or that would even make it possible to reduce sensitivity to this risk.
Additional attempts have been made to define a method that makes it possible to fabricate welded blanks from sheets of different thicknesses that would not result in a significantly different heating rate in the different parts of these welded blanks.
Finally, attempts have been made to devise a method in which the constituent steps and the physical elements make the implementation of the method possible without resulting in prohibitive cost increases.